Jo22 wrote on 2021-05-09, 03:18:

This reminds me of an episode on The 8-Bit Guy's channel.

https://www.youtube.com/watch?v=NkpSBK3g-gA

That's very interesting! I've seen almost all the videos from his channel, but missed this one.
It's amazing that such a simple circuit works so well!

mothergoose729 wrote on 2021-05-08, 20:28:

In DOS, most games run at 70hz and 400p. The good news is that DOS games actually only have about 200 lines of detail and are great candidates for downscaling, but that 70hz thing can definitely be a problem. There are apparently VGA cards that support 15khz natively and that might work, but I am worried about how the 70hz->60hz conversion would work on external scalers. Make sure those features are documented before you buy something expensive.

The 70Hz are the smallest problem. Adjust the framing and you are done.
Setting the registers for vertical total and vertical sync position to appropriate values should do the trick and leave you with 60Hz.

maxtherabbit wrote on 2021-05-08, 22:39:

What kind of VGA card do you have? Give VGATV a shot.

Thank you, but I have a couple PCs and I doubt all my cards are compatible (I'm not sure any of them is, in fact). Currently testing with a 386 that has a Realtek RTG3105, but I have a much newer PC with a Geforce 2 MX, and a Pentium MMX system waiting to be assembled (also with a very generic cheap gfx card).

I've also just found a Trident TVGA9000B card that I didn't know I had, but it doesn't work - severe oxidation, and cleaning it didn't help (with this card the PC is beeping a code I can't find the meaning for, gotta find my ISA POST card).

maxtherabbit wrote on 2021-05-08, 19:25:

No need for a transcoder, the 1084 supports analogue RGB natively.

There is a DOS TSR called VGATV that can force supported VGA cards into outputting 15kHz RGBHV. It works pretty well if you have a supported card. I use it on a S3 ViRGE, ET4000, and Trident 8900 and it works great.

very much this!
I ran my first 386 PC connected to living room 21' Daewoo TV this way. VGA to SCART cable + TRS loaded to reprogram CRTC for 15KHz. 320x200 and text modes came out razor sharp. I finished Doom 1/2, Ufo 1/2, Colonization, Privateer and few other games on this setup before buying my first real VGA monitor. I later used same cable on second computer under Windows with Commodore 1084S Amiga monitor to play Diablo over lan with mixed results, interlaced 640x480 and I think I had to use powerstrip? to manually sync it.

Search for "VGATV for DOS driver", "CRT Emudriver" or "Soft-15KHz" for modern windows versions.

Btw my VGA to EURO SCART cable came from a shady vendor at the local computer market (1993?1994?). Contained dip14 (edit: sanded off) chip handling sync inside Epoxy filled plug (hardware obfuscation), but I made my own copy just joining syncs together without resistors and that also worked.

rasz_pl wrote on 2021-05-09, 11:57:

Btw my VGA to EURO SCART cable came from a shady vendor at the local computer market (1993?1994?). Contained dip14 chip handling sync inside Epoxy filled plug (hardware obfuscation), but I made my own copy just joining syncs together without resistors and that also worked.

Could that have been an XOR chip, i.e. the proper way to join the sync signals? The 7486 and 4070 come to mind.
Joining the sync signals without resistors sounds dangerous.

I believe the 1084 supports separate syncs. The sync combiner "circuits" discussed here would not be necessary.

There is no way to properly combine sync with basic combinational logic, but XNOR is the best of the worst.

Chip in my cable was sanded off, and turned out unnecessary at the end.

rasz_pl wrote on 2021-05-09, 15:42:

Chip in my cable was sanded off, and turned out unnecessary at the end.

Oh, how I hate such stuff. 😑 Using custom stickers is one thing, but sanding off the chip markings is just plain evil and selfish.
I know this from my ham radio hobby. In the past, some egoistic guys sold products that way.
They didn't want others to replicate their creations.. People like this totally ruin the hobby for commerce.
On top of that, this makes fixing stuff you paid for difficult.
Serious people don't do practice that, only wimps do.
Any company with at least a bit of reputation and integrity even adds a basic schematic in the manual.

Benedikt wrote on 2021-05-09, 13:54:

Could that have been an XOR chip, i.e. the proper way to join the sync signals? The 7486 and 4070 come to mind.
Joining the sync signals without resistors sounds dangerous.

maxtherabbit wrote on 2021-05-09, 14:59:

I believe the 1084 supports separate syncs. The sync combiner "circuits" discussed here would not be necessary.

There is no way to properly combine sync with basic combinational logic, but XNOR is the best of the worst.

That's slightly off-topic perhaps, but personally speaking, I came to the conclusion,
that quick&dirty/temporary solutions are sometimes longer lasting than the real thing. 😁
('nothing is more definitive than the temporary'.)

Also, the simple, hacky solutions not seldomly do cause less trouble on the long run.
For example, passive ciruits usually do not introduce so much noise (hi transistors, ICs, switching PSUs).
I'm speaking under correction, but I believe in case of an SCART RGB plug,
for example, using a little 1. xV battery (coin cell?) for the Composite/RGB switch signal might be unconventional,
but less dangerous/noisy than doing it “properly“ with a 5v or 12v signal that goes through a DC step down converter (1 to 3v out?).

Same goes for all these Composite mods for old consoles:
Yes, RF is noisy by comparison, but sometimes there's a much easier fix to the issue.
Like, for example, using a clean power source, like a 9v battery/rechargeable, instead of a cheap power brick (modern switching PSUs or old unstabilized bricks with just a trafo) .

Speaking of rechargeables, there's something so incredible simple, yet elegant.
Instead of a modern, over-engineered charging regulator
with a microcontroller, a simple lamp of the same voltage will also do. 🙂
If installed in series with a power source, it will protect both the rechargeable battery and the power source (acts as a fuse, even).
But that's not all, it will also work as a visible indicator. If the lamp stops glowing, the rechargeable battery is fully charged.

VioletGiraffe wrote on 2021-05-09, 08:14:

Jo22 wrote on 2021-05-09, 03:18:

This reminds me of an episode on The 8-Bit Guy's channel.

https://www.youtube.com/watch?v=NkpSBK3g-gA

That's very interesting! I've seen almost all the videos from his channel, but missed this one.
It's amazing that such a simple circuit works so well!

Hi again! I've just discovered another video from him, in which he talks about the qualities of purw
monochrome monitors and in which he explains the fuzzy picture via Composite..

https://www.youtube.com/watch?v=BJzOErvJwZs

By the way, display technology still has a long way to go.
It's 2021 and we're still using RGB. 🙁
Ideally, a monitor would have a single pixel that could change to millions of different colours, just like monochrome technology used to have.
The current method of using three or four sub pixels (red/green/blue/yellow) is not very advanced.
It leds to moire patterns, poor black values and a limited viewing angle.

Back in the 70s/80s there were experiments with monochrome screens and coloured layers that gave much more vibrant colours (Nu Color, LCCS etc).

https://www.youtube.com/watch?v=z-q8ehzHeQQ

https://www.youtube.com/watch?v=LaK9MIYm6J8

Also, in astronomy, cameras are monochromatics and use filters for various wave lenghts.

https://www.youtube.com/watch?v=lD0ZH2doSlk

Anyway, I just felt I should mention this.
Maybe in the future, when we're all using laser TVs or proper OLED screens,
color e-ink displays or what not, people will realize that we had quite a bit of imagination and foresight. 😉

I had VGATV working back in the day with a 1024S through the 9 pin input, also had a Hitachi TV that had RGB inputs on a 7 pin DIN... Using CL542x cards mostly on the PC end... The "adapter" got to be some kind of crazy octopus thing, since as well as sync combine, it got sync separator, and VGA plug and socket, so I could connect Amiga or PC to any display device I had at the time. Amiga to VGA or PC to TV etc...

Now of interest to y'all, would possibly be that I've found RGB lines broken out INSIDE LCD DVD headrest monitors for vehicles, you know the strap on the back type that go with a portable DVD player etc for in car movies for the back seat ppl. Anyhoos... pik one with a 4:3 or aspect switch/button and you can have your retro gaming box run VGATV and wire it up to one of those little buggers for 5 to 10 inch color display for the lower res games. If you get lucky with the chipset in a mono display 286/386/486 laptop or clamshell, you could make a matching unit to mount over or on top of your mono screen for color (and faster update, most of them aren't too bad for that or movies looked sucky) gaming. Also you can just try the CGA composite outputs into them if you've got those.